So I have a programming assignment where they supply us with the header file which contains:
class SEQ{
private:
...// stuff
public:
...//constructor
string operator ( ) ( ) {
string tmp = id; int i = nd - 1;
if ( id [ i ] < '9' ) id [ i ]++;
else {
id [ i ] = '1'; bool flag = true;
for ( i--; i >= 0 && flag; i-- )
if ( id [ i ] < 'Z' ) { id [ i ]++; flag = false; }
else id [ i ] = 'A';
}
return tmp;
}
}
I am supposed to use this with std::generate on an std::vector. However, the function never gets called. I have tried writing it these ways:
vector<string> v;
for (int i = v.size(); i < in.N; i++) v.push_back("");
generate(v.begin(), v.end(), SEQ); // compiler error
generate(v.begin(), v.end(), SEQ()); // no error but does nothing
SEQ seq; // declare an instance
generate(v.begin(), v.end(), seq); // no error but does nothing
generate(v.begin(), v.end(), seq()); // compiler error algorithm line 1527 does not evaluate to a function that takes 0 arguments
I am assuming the header file they gave me does not contain any errors. How am I supposed to write this so that the function gets called?
generate takes a function pointer or a reference to an instance of a functor. The following two calls are correct. They are no-ops if the vector is empty.
// pass a default-constructed temporary instance
generate(v.begin(), v.end(), SEQ{});
// pass a named instance
SEQ seq;
generate(v.begin(), v.end(), seq);
The following works:
#include <algorithm>
#include <vector>
#include <cassert>
int main()
{
std::vector<size_t> v(10);
auto seq = [a = 0]() mutable { return a++; };
std::generate(v.begin(), v.end(), seq);
assert(v.begin() != v.end());
for (size_t i = 0; i < v.size(); ++i)
assert(v[i] == i);
}
Related
I declared a class like this:
class myclass{
array myarr;
pair<PT3D,PT3D> mypair;
ID myid;
CString tag;
}
after fill data, i have a vector<myclass> myvec;
How can I sort this vector based on the CString value of each class?
bool SortCompare(const wchar_t* a, const wchar_t* b)
{
if (wcslen(a) == wcslen(b))
{
int k = (int)wcslen(a);
return std::wcsncmp(a, b, k);
}
else
return wcslen(a) < wcslen(b);
}
sort(myvec.begin(), myvec.end(), [h](myclass& a,
myclass& b) {
return SortCompare(a.tag, b.tag);
});
However this doesn't work properly , my desired result is a list sorted by human sort as this post.
but I don't know how to use CSortStringArray::CompareAndSwap function, how to use this code with std::sort?
How can I correct my codes?
Note CString provides less operator so you do not have to fall back to C-API like wcsncmp.
You are making this overcomplicated.
using C++11:
std::sort(myvec.begin(), myvec.end(), [](const myclass& a, const myclass& b) {
return a.tag < b.tag;
});
C++20 has even something simpler called projection:
std::ranges::sort(myvec, {}, &myclass::tag);
Now the issue of alphanumeric compare. This can look like this:
struct AlphanumericSplitResult {
std::string_view prefix;
std::string_view digits;
std::string_view suffix;
};
AlphanumericSplitResult alphanumericSplit(std::string_view s)
{
auto i = s.find_first_of("0123456789"sv);
i = i == std::string_view::npos ? s.size() : i;
auto j = s.find_first_not_of("0123456789"sv, i);
j = j == std::string_view::npos ? s.size() : j;
return { s.substr(0, i), s.substr(i, j - i), s.substr(j) };
}
bool asNumberLess(std::string_view l, std::string_view r)
{
return l.size() != r.size() ? l.size() < r.size() : l < r;
}
bool alphanumeric_less(std::string_view l, std::string_view r)
{
if (l.empty())
return !r.empty();
auto a = alphanumericSplit(l);
auto b = alphanumericSplit(r);
if (a.prefix != b.prefix)
return a.prefix < b.prefix;
if (a.digits != b.digits)
return asNumberLess(a.digits, b.digits);
return alphanumeric_less(a.suffix, b.suffix);
}
It passes all test I wrote: https://godbolt.org/z/hdG6fq9sa
and it is easy to feed to both algorithms.
Disclaimer: there is small bug in implementation - can you find it? Depending on requirements there are more small issues.
std::wcsncmp returns 0, > 0 or < 0. > 0 and < 0 are converted to true, 0 to false. Thus, compare "a" with "b" and "b" with "a" both return true. std::sort expects a proper compare function. Perhaps you want return std::wcsncmp(a, b, k) < 0.
In your else, you return Len(a) < Len( b) but this is not the same as Len(a) != Len(b). I would just return false in the else block since you know they aren't the same at that point.
#include <bits/stdc++.h>
using namespace std;
#include <unordered_set>
#include <queue>
struct word {
string s;
int level;
word(string a, int b)
: s(a)
, level(b)
{
}
};
bool isadj(string s1, string s2)
{
int len = s1.length(), count = 0;
for (int i = 0; i < len; i++) {
if (s1[i] != s2[i])
count++;
if (count > 1)
return false;
}
return count == 1 ? true : false;
}
int ladderLength(string beginWord, string endWord, vector<string>& wordList)
{
unordered_set<string> st;
for (string s : wordList)
st.insert(s); // adding elements into a set
if (st.find(endWord) == st.end())
return 0;
queue<word> q;
q.push(word(beginWord, 0)); // initialising the queue
while (!q.empty()) {
word temp = q.front(); // pop the current string
q.pop();
if (temp.s == endWord)
return temp.level;
for (auto it = st.begin(); it != st.end(); it++) { // loop over the set to find strings at a distance of 1 and add them to the queue
if (isadj(temp.s, *it)) // i have inserted code here to print the string *it
{
q.push(word(*it, temp.level + 1));
st.erase(*it); // delete the element to avoid looping
}
}
}
return 0;
}
int main()
{
// make dictionary
vector<string> D;
D.push_back("poon");
D.push_back("plee");
D.push_back("same");
D.push_back("poie");
D.push_back("plie");
D.push_back("poin");
D.push_back("plea");
string start = "toon";
string target = "plea";
cout << "Length of shortest chain is: "
<< ladderLength(start, target, D);
return 0;
}
The problem i am trying to solve is https://leetcode.com/problems/word-ladder/
I am unable to trace where I am using a memory that was deallocated again in my program?
The following are my attempts to debug :
I tried to run it on another online ide where the code compiles and runs successfully but gives a wrong answer . in order to debug it I have inserted some lines into my code in order to print all the strings which are at a distance of 1 for my current string. surprisingly an empty string is appearing to be in the set. Please help me in understanding where am I doing a mistake.
unordered_set::erase returns a value, and this returned value is important. You should not ignore it.
In your case, once you erase something from the set, it is invalid. Trying to increment it results in Undefined Behavior.
The correct approach is to replace the current iterator with the returned one, then not increment during the loop.
for (auto it = st.begin(); it != st.end(); )
if (...) {
// ...
it = st.erase(*it);
} else
++it;
After the line:
st.erase(*it); // delete the element to avoid looping
the it iterator is not valid and should not be used.
Your problem seems to be already addressed, but if you'd be interested, this'd also pass without using std::queue, only using std::unordered_set:
// The following block might slightly improve the execution time;
// Can be removed;
static const auto __optimize__ = []() {
std::ios::sync_with_stdio(false);
std::cin.tie(NULL);
std::cout.tie(NULL);
return 0;
}();
// Most of headers are already included;
// Can be removed;
#include <cstdint>
#include <string>
#include <vector>
#include <unordered_set>
#include <algorithm>
using ValueType = std::int_fast16_t;
static const struct Solution {
static const int ladderLength(
const std::string start,
const std::string end,
const std::vector<std::string>& words
) {
std::unordered_set<std::string> words_map(std::begin(words), std::end(words));
std::unordered_set<std::string> head;
std::unordered_set<std::string> tail;
std::unordered_set<std::string>* curr_head;
std::unordered_set<std::string>* curr_tail;
if (words_map.find(end) == std::end(words_map)) {
return 0;
}
head.insert(start);
tail.insert(end);
ValueType ladder = 2;
while (!head.empty() && !tail.empty()) {
if (head.size() < tail.size()) {
curr_head = &head;
curr_tail = &tail;
} else {
curr_head = &tail;
curr_tail = &head;
}
std::unordered_set<std::string> temp_word;
for (auto iter = curr_head->begin(); iter != curr_head->end(); iter++) {
std::string word = *iter;
for (ValueType index_i = 0; index_i < word.size(); index_i++) {
const char character = word[index_i];
for (ValueType index_j = 0; index_j < 26; index_j++) {
word[index_i] = 97 + index_j;
if (curr_tail->find(word) != curr_tail->end()) {
return ladder;
}
if (words_map.find(word) != std::end(words_map)) {
temp_word.insert(word);
words_map.erase(word);
}
}
word[index_i] = character;
}
}
ladder++;
curr_head->swap(temp_word);
}
return 0;
}
};
You might want to break it into more methods, a bit too long for a function.
References
For additional details, please see the Discussion Board where you can find plenty of well-explained accepted solutions with a variety of languages including low-complexity algorithms and asymptotic runtime/memory analysis1, 2.
i have given a vector `
vector<string> inputArray = { "aba","aa","ad","vcd","aba" };
and i want to return this vector which contains only string with the longest length, in this case i want to return only {"aba","vcd","aba"}, so for now i want to erase elements which length is not equal to the highest `
vector<string> allLongestStrings(vector<string> inputArray) {
int length = inputArray.size();
int longstring = inputArray[0].length();
int count = 0;
vector<string> result;
for (int i = 0; i < length; i++)
{
if (longstring < inputArray[i].length())
{
longstring = inputArray[i].length();
}
count++;
}
for (int = 0; i<count;i++)
{
if (inputArray[i].length() != longstring)
{
inputArray[i].erase(inputArray.begin() + i);
count--;
i--;
}
}
return inputArray;
}
but i get this error no instance of overloaded fucntion "std::basic_string<_Elem,_Traits,_Alloc>::erase[with_Elem=char,_Traits=std::char_traits<char>,_Alloc=std::allocator<char>]" matches the argument list" in inputArray[i].erase(inputArray.begin()+i); this line
what's wrong?
There are other problems, but this specific compiler message is telling you that's not the right way to remove specific character(s) from a string.
However, reading the question in the OP, we see that you wanted to remove a string from a vector. To fix that one specific error, simply change
inputArray[i].erase( /*character position(s) in the string*/ )
to
inputArray.erase( /*some position in the array*/ )
Or you could fix it so it uses an iterator in the string denoted by inputArray[i] to actually delete characters from that string, which of course isn't what you said you wanted to do. The point is, the error message is because you're using the wrong iterator type because you think that you're working with a vector, but you actually told it to work with a string that you got out of the vector.
And then you will compile and have other issues which are well covered in comments already.
The issue with inputArray[i].erase(inputArray.begin() + i); can be fixed as shown in Kenny Ostrom's answer.
I'd like to point out that the OP could make use of the erase-remove idiom or even create a new vector with only the bigger strings instead (the posted code is already copying the source vector).
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
template <typename InputIt>
auto only_the_longest_of(InputIt first, InputIt last)
{
using value_type = typename std::iterator_traits<InputIt>::value_type;
std::vector<value_type> result;
// find the longest size
auto longest = std::max_element(first, last,
[](value_type const &a, value_type const &b) {
return a.size() < b.size();
});
if ( longest == last )
return result;
// extract only the longest ones, instead of erasing
std::copy_if( first, last, std::back_inserter(result)
, [max_size = longest->size()] (value_type const& v) {
return v.size() >= max_size;
});
return result;
}
template <typename T>
auto erase_the_shortest_from(std::vector<T> &input)
{
// find the longest size
auto longest = std::max_element(input.cbegin(), input.cend(),
[](T const &a, T const &b) {
return a.size() < b.size();
});
if ( longest == input.cend() || longest->size() == 0 )
return input.end();
// implement erase-remove idiom
return input.erase(std::remove_if(
input.begin(), input.end(), [max_size = longest->size()] (T const &v) {
return v.size() < max_size;
}));
}
int main()
{
std::vector<std::string> test = {
"aba", "aa", "ad", "vcd", "aba"
};
// The original vector remain unchanged
auto result = only_the_longest_of(test.cbegin(), test.cend());
for (auto const& str : result)
std::cout << str << '\n';
std::cout << '\n';
// This will change the vector
erase_the_shortest_from(test);
for (auto const& str : test)
std::cout << str << '\n';
}
I have to process an std::vector either forwards or in reverse, depending upon a boolean flag. What's the most elegant way to accomplish this? Before needing to do it in reverse I had:
BOOST_FOREACH(const CType &foo, vec) {
...
}
However, now I have the horrendous-looking:
for (int i=undoing ? (vec.size()-1) : 0; undoing ? (i >= 0) : (i < vec.size()); undoing ? (i--) : (i++)) {
const CType &foo = vec[i];
...
}
Is there a better way?
I don't know that people would call it elegant, but there's:
auto do_it = [](const CType& elem)
{
...
};
if (iterate_forward) {
std::for_each(vec.begin(), vec.end(), do_it);
}
else {
std::for_each(vec.rbegin(), vec.rend(), do_it);
}
Add a template function that works with either the forward iterators or reverse iterators. Call the function using the appropriate iterator based on the value of undoing.
template <typename Iterator>
void doStuff(Iterator iter, Iterator end)
{
for ( ; iter != end; ++iter )
{
// Do stuff
}
}
if ( undoing )
{
doStuff(vec.rbegin(), vec.rend());
}
else
{
doStuff(vec.begin(), vec.end());
}
How about keeping the loop running as it is from 0 to vector.size, but reading the array in the direction you need.
int idx;
for (int i =0; i < vec.size(); i ++)
{
if (undoing) // assuming going forward
idx = i;
else // going backwards
idx = vec.size() - i - 1;
const CType &foo = vec[idx];
}
You may also use Boost.Range-based solution. It's similar to the one using STL algorithms, already proposed.
#include <boost/range/adaptor/reversed.hpp>
#include <boost/range/algorithm/for_each.hpp>
// In C++11 lambda expression can be used instead
struct my_fun
{
void operator()(const CType& elem) const
{
/*...*/
}
};
/*...*/
using namespace boost::adaptors;
if ( iterate_forward )
boost::for_each(my_vect, my_fun());
else
boost::for_each(my_vect | reversed, my_fun());
My question is related to this.
I wanted to perform a sort() operation over the set with the help of a lambda expression as a predicate.
My code is
#include <set>
#include <string>
#include <iostream>
#include <algorithm>
int main() {
using namespace std;
string s = "abc";
set<string> results;
do {
for (int n = 1; n <= s.size(); ++n) {
results.insert(s.substr(0, n));
}
} while (next_permutation(s.begin(), s.end()));
sort (results.begin(),results.end());[](string a, string b)->bool{
size_t alength = a.length();
size_t blength = b.length();
return (alength < blength);
});
for (set<string>::const_iterator x = results.begin(); x != results.end(); ++x) {
cout << *x << '\n';
}
return 0;
}
But the numbers and types of errors were so complex that I couldn't understand how to fix them. Can someone tell me whats wrong with this code.
Edit: Note that Steve Townsend's solution is actually the one you're searching for, as he inlines as a C++0x Lambda what I write as C++03 code below.
Another solution would be to customize the std::set ordering function:
The std::set is already ordered...
The std::set has its own ordering, and you are not supposed to change it once it is constructed. So, the following code:
int main(int argc, char* argv[])
{
std::set<std::string> aSet ;
aSet.insert("aaaaa") ;
aSet.insert("bbbbb") ;
aSet.insert("ccccccc") ;
aSet.insert("ddddddd") ;
aSet.insert("e") ;
aSet.insert("f") ;
outputSet(aSet) ;
return 0 ;
}
will output the following result:
- aaaaa
- bbbbb
- ccccccc
- ddddddd
- e
- f
... But you can customize its ordering function
Now, if you want, you can customize your set by using your own comparison function:
struct MyStringLengthCompare
{
bool operator () (const std::string & p_lhs, const std::string & p_rhs)
{
const size_t lhsLength = p_lhs.length() ;
const size_t rhsLength = p_rhs.length() ;
if(lhsLength == rhsLength)
{
return (p_lhs < p_rhs) ; // when two strings have the same
// length, defaults to the normal
// string comparison
}
return (lhsLength < rhsLength) ; // compares with the length
}
} ;
In this comparison functor, I did handle the case "same length but different content means different strings", because I believe (perhaps wrongly) that the behaviour in the original program is an error. To have the behaviour coded in the original program, please remove the if block from the code.
And now, you construct the set:
int main(int argc, char* argv[])
{
std::set<std::string, MyStringLengthCompare> aSet ;
aSet.insert("aaaaa") ;
aSet.insert("bbbbb") ;
aSet.insert("ccccccc") ;
aSet.insert("ddddddd") ;
aSet.insert("e") ;
aSet.insert("f") ;
outputSet(aSet) ;
return 0 ;
}
The set will now use the functor MyStringLengthCompare to order its items, and thus, this code will output:
- e
- f
- aaaaa
- bbbbb
- ccccccc
- ddddddd
But beware of the ordering mistake!
When you create your own ordering function, it must follow the following rule:
return true if (lhs < rhs) is true, return false otherwise
If for some reason your ordering function does not respect it, you'll have a broken set on your hands.
std::sort rearranges the elements of the sequence you give it. The arrangement of the sequence in the set is fixed, so the only iterator you can have is a const iterator.
You'll need to copy results into a vector or deque (or such) first.
vector sortable_results( results.begin(), results.end() );
You can customize the ordering of the elements in the set by providing a custom predicate to determine ordering of added elements relative to extant members. set is defined as
template <
class Key,
class Traits=less<Key>,
class Allocator=allocator<Key>
>
class set
where Traits is
The type that provides a function
object that can compare two element
values as sort keys to determine their
relative order in the set. This
argument is optional, and the binary
predicate less is the default
value.
There is background on how to use lambda expression as a template parameter here.
In your case this translates to:
auto comp = [](const string& a, const string& b) -> bool
{ return a.length() < b.length(); };
auto results = std::set <string, decltype(comp)> (comp);
Note that this will result in set elements with the same string length being treated as duplicates which is not what you want, as far as I can understand the desired outcome.
sort requires random access iterators which set doesn't provide (It is a bidirectional iterator). If you change the code to use vector it compiles fine.
You cannot sort a set. It's always ordered on keys (which are elements themselves).
To be more specific, std::sort requires random access iterators. The iterators provided by std::set are not random.
Since I wrote the original code you're using, perhaps I can expand on it... :)
struct cmp_by_length {
template<class T>
bool operator()(T const &a, T const &b) {
return a.length() < b.length() or (a.length() == b.length() and a < b);
}
};
This compares by length first, then by value. Modify the set definition:
set<string, cmp_by_length> results;
And you're good to go:
int main() {
using namespace std;
string s = "abc";
typedef set<string, cmp_by_length> Results; // convenience for below
Results results;
do {
for (int n = 1; n <= s.size(); ++n) {
results.insert(s.substr(0, n));
}
} while (next_permutation(s.begin(), s.end()));
// would need to add cmp_by_length below, if I hadn't changed to the typedef
// i.e. set<string, cmp_by_length>::const_iterator
// but, once you start using nested types on a template, a typedef is smart
for (Results::const_iterator x = results.begin(); x != results.end(); ++x) {
cout << *x << '\n';
}
// of course, I'd rather write... ;)
//for (auto const &x : results) {
// cout << x << '\n';
//}
return 0;
}
std::set is most useful to maintain a sorted and mutating list. It faster and smaller to use a vector when the set itself wont change much once it's been built.
#include <vector>
#include <string>
#include <iostream>
#include <algorithm>
int main() {
using namespace std;
string s = "abc";
vector<string> results;
do {
for (size_t n = 1; n <= s.size(); ++n) {
results.push_back(s.substr(0, n));
}
} while (next_permutation(s.begin(), s.end()));
//make it unique
sort( results.begin(), results.end() );
auto end_sorted = unique( results.begin(), results.end() );
results.erase( end_sorted, results.end() );
//sort by length
sort (results.begin(),results.end());
[](string lhs, string rhs)->bool
{ return lhs.length() < rhs.length(); } );
for ( const auto& result: results ) {
cout << result << '\n';
}
}
I used the classic, sort/unique/erase combo to make the results set unique.I also cleaned up your code to be a little bit more c++0x-y.